In recent years, glass products used in the industry of optical communication, semiconductors, etc. are under very severe controls with respect to the purity of the constituting glass materials. To produce such a highly pure glass product, there are mainly the following methods: (1) a method using sand-like natural quartz powder (what is called "sand") obtained by pulverizing natural quartz; to obtain more purified ones, (2) oxygen-hydrogen flame method, a method which comprises using a lump of fume, obtained by adhesion and growth on the substrate, of the fume generated by the hydrolysis of silicon tetrachloride in the oxygen-hydrogen flame; (3) a method using a gel obtained from an organic metal compound such as alkoxysilane as a starting material, what is called solgel method.
However, these methods all have their merits and demerits. (1) When natural quartz is used as starting material, metal elements e.g. aluminum, iron, are included essentially inside quartz particle, and it is difficult to purify to the level of not more than 10 ppb even by repeating purification such as acid washing. (2) By hydrogen oxygen flame method, using tetrachlorosilane, high purification can be sought. However, as cost has difficulty to compensate industrially, mass production is not achieved.
On the other hand, (3) sol-gel method needs low cost hence mass production can be sought, but quartz glass obtained by sol-gel method generally contains fine bubbles said to derive from silanol, causing problems especially in semiconductor industries e.g. photomask, crucible for single silicon crystal drawing, and also in optical communication fields such as optical fiber causing problems such as optical loss during fiber elongation and during use and bad effects due to gas in the bubbles.
Also, in sol gel method, synthetic quartz glass powder obtained from a gel treated by steam in advance before drying, or a gel washed with water before calcination, can reduce in some degree the bubbling in the shaped glass product after fusing. However, such steam treatment needs a long time, and also there is a problem that, because the gel contains large amount of water, mechanical strength of the gel is weak, and tends to collapse and pulverize during the steam treatment. Also, relating to the water treatment before calcination, repetition of process such as the dried gel is again washed with water and dried, causes waste of energy and problem of severe cost up. As described heretofore, by the conventional methods, there are problems both technically and in view of cost, and in addition to that, the degree of preventing bubbling is not enough, so also from this point of view, these methods couldn't be used in industrial operation.